Biochemistry Chapter 18: Oxidative Decarboxylation of Pyruvate

What are the fates of pyruvate?

anaerobic conditions- lactic acid/ethanol through fermentation; active muscle

aerobic conditions- acetyl coenzyme A through the citric acid cycle; resting muscle

How is pyruvate linked to the citric acid cycle?

Pyruvate is oxidatively decarboxylated to acetyl CoA, then transported from the cytoplasm into the mitochondrial matrix.

E1 of pyruvate dehydrogenase complex

pyruvate dehydrogenase, TPP prosthetic group, catalyzes oxidative decarboxylation of pyruvate

E2 of pyruvate dehydrogenase complex

dihydrolipoyl transacetylase, lipoamide prosthetic group, transfers the acetyl group to CoA

E3 of pyruvate dehydrogenase complex

dihydrolipoyl dehydrogenase, FAD prosthetic group, reoxidizes lipoamide and produces NADH

What is the structure of the pyruvate dehydrogenase complex?

E2 core, E1 and E3 outer shell

coenzymes required for synthesis of acetyl CoA

catalytic coenzymes- TPP, lipoamide, FAD

stoichiometric coenzymes- CoA, NAD+

Step 1- decarboxylation of pyruvate

E1 pyruvate dehydrogenase

Step 2- oxidation of hydroxyxethyl group to acetyl group

E1 pyruvate dehydrogenase

Step 3- transfer of acetyl group from lipoamide to CoA

E2 dihydrolipoyl transacetylase

Regeneration Step- oxidation of dihydrolipoamide, formation of NADH

E3 dihydrolipoyl dehydrogenase

structure of E2 transacetylase

• hollow cube of 20 catalytic trimers, each with three major domains

• amino terminus (lipoamide arm), small domain, large transacetylase domain

lipoamide arm

long, flexible arm which carries substrates from one active site to the next

E3 binding protein

essential protein contained in the core which facilitates interaction between E2 and E3, activity is greatly reduced without it

What are the benefits of a integrated enzyme complex?

increases reaction rate, minimizes side reactions, intermediates remain bound to complex and are readily transferred

What is the significance of turning pyruvate into acetyl CoA?

The irreversible reaction commits the carbons of glucose to either oxidation to CO₂ (CAC) or incorporation into fatty acids (fatty acid synthesis).

inhibitors of pyruvate dehydrogenase complex

• high concentrations of reaction products, acetyl CoA and NADH

• high ATP signals high energy charge

How is the pyruvate dehydrogenase complex regulated?

• reversible phosphorylation

• pyruvate dehydrogenase kinase- terminates activity by phosphorylating E1

• pyruvate dehydrogenase phosphatase- stimulates activity by reversing phosphorylation

stimulators of pyruvate dehydrogenase complex

• high ADP and pyruvate indicate low energy charge

• high Ca²⁺ to stimulate muscle contraction

• epinephrine (liver), insulin (liver, adipose tissue) indicate high energy demand